Polyamides, more commonly referred to as nylons, are thermoplastic materials which feature repeating amide groups. They are generally semi-crystalline, have superior load-bearing capability at elevated temperatures, good toughness, low frictional properties, and good chemical resistance. Amorphous polyamide resins are transparent, although they can be tinted or colored opaquely. They are frequently used in applications requiring transparency, toughness, and chemical resistance. In addition, while most nylons are inherently hygroscopic, amorphous nylons have substantially lower water absorption levels at equilibrium.
In recent years, nylons have been blended with various homopolymers and copolymers to yield products which have improved properties for their particular use. For example, alloys of nylon with olefin copolymers yield high-performance injection molding compounds with improved toughness and lowered water absorption. Blends of crystalline nylons with sulfone polymers are disclosed in Hartsing, Jr., U.S. Pat. No. 4,503,168 in order to enhance the solvent resistance of the sulfone polymers. Amorphous polyamides have been improved by the addition of various impact modifiers when additional impact strength is desired. Such impact modifiers are typically natural or synthetic rubber compounds including polymeric and copolymeric compounds such as butadiene rubber, EPDM rubber, di-and tri-block copolymers of styrene and butadiene, as well as other elastomeric products. Many of these are commercially available from a variety of sources.
Poly(alkenyl aromatic) resins such as polystyrene are thermoplastic resins having a wide range of physical properties. They are available in formulations ranging from general purpose crystal polystyrene to impact grades and other highly specialized resins modified for foam molding and extrusion, flame-retardation, etc. The wide range of physical properties obtainable with the poly(alkenyl aromatic) resins makes them a desirable material. In addition, it is known that rubber-modified poly(alkenyl aromatic) resins are useful thermoplastic materials which are relatively low-cost. Such resins have become widely used in spite of some deficiencies in impact strength, thermal resistance and surface appearance, e.g., gloss in thermoformed articles.
In recent years, polystyrenes have been blended with various copolymers to yield thermoplastic compositions which have improved physical properties over that of polystyrene alone. For example, Cizek, U.S. Pat. No. 3,383,435 discloses blends of polyphenylene ethers and styrene resins, which are said to have improved resistance to organic solvents. Cizek further discloses that blends of a polyphenylene ether and a styrene-acrylonitrile (SAN) copolymer have even superior chemical resistance than polyphenylene ether/polystyrene blends. Yu et al., U.S. Pat. No. 3,944,631 discloses styrene-acrylonitrile copolymers which are modified by adding an impact modifier consisting of a cross-linked (meth)acrylate rubbery core and a crosslinked interpenetrating styrene-acrylonitrile resin shell in order to improve impact strength and weather resistance. Other useful thermoplastic compositions are disclosed in Katchman, U.S. Pat. No. 4,341,882 wherein polyphenylene ethers and alkenyl aromatic resins are combined with astyrene-allyl alcohol copolymer as an anti-static agent.
Mention is also made of blends of reactive polystyrenes with thermoplastic resins, specifically, polyphenylene ether resins For example, Hohlfeld, U.S. Pat. No. 4,590,241 reacts styrene with an oxazoline compound then blends the product with polyphenylene ethers and low density polyethylenes to form miscible blends which behave in an intermediate fashion as compared to the individual components.
Many other styrene-based blends are well-known in the art, such as polystyrene with block copolymers of polystyrene and an elastomer such as polyisoprene, polybutadiene, ethylene-propylene, or ethylene-butylene rubber.
However, the uses of polystyrene have been limited due to its susceptibility to degradation from UV radiation, and to chemical attack. While impact modification of polystyrene can provide a tensile and flexural strength which can range from 2000 to 7000 p.s.i., the impact grades are also susceptible to UV radiation, and offer limited chemical resistance.
It has now been discovered that blends of amorphous polyamides with poly(alkenyl aromatic) resins have improved properties over previously known blends, including producing surprisingly clear, optically transparent materials, having near zero birefringence, over their entire compositional range. Because of these improved properties and transparency, blends of amorphous polyamides and poly(alkenyl aromatic) resins are suitable for many commercial uses, such as optical devices. These blends are immiscible, as determined by the presence of two glass transition temperatures.